Cascaded Amplification and Manipulation of Terahertz Emission by Flexible Spintronic Heterostructures

Spintronic heterostructures consisting of ferromagnetic (FM) and nonmagnetic (NM) have become increasingly important devices for terahertz (THz) pulse generation, named as spintronic THz emitter (STE). The recycling of the laser pump energy is known to have a key impact on further improvement of the THz emission. Here, an efficient and practical approach to enhance and manipulate THz generation based on a cascaded Pt/CoFe/Ta trilayer fabricated on a flexible polyethylene terephthalate (PET) substrate is shown. Considering the refractive index n of PET is ≈1.61 at 1.55 eV and ≈1.75 at THz frequencies, it is anticipated that the femtosecond (fs) laser pulse propagates with a similar group velocity as the generated THz pulse. Therefore, the cascaded design enables all THz emission from each PET/STE propagating almost in-phase and yields a 1.55 times amplification compared to a single PET/STE. Two proof-of-concept demonstrations are experimentally presented. First one, a metasurface can be assembled with the cascaded PET/STE to manipulate the THz signal. Second one, the cascaded PET/STE is used to perform a spectroscopic measurement of riboflavin. These demonstrations highlight the potential of flexible PET/STE for building blocks of advanced functionalities, such as compact THz emitters, THz spectroscopic, and imaging systems.